Debugger#

Stability: 2 - Stable

Node.js includes a command-line debugging utility. The Node.js debugger clientis not a full-featured debugger, but simple stepping and inspection arepossible.

To use it, start Node.js with theinspect argument followed by the path to thescript to debug.

$node inspect myscript.js< Debugger listening on ws://127.0.0.1:9229/621111f9-ffcb-4e82-b718-48a145fa5db8< For help, see: https://nodejs.org/en/docs/inspector<connecting to 127.0.0.1:9229 ... ok< Debugger attached.< okBreak on start in myscript.js:2  1 // myscript.js>2 global.x = 5;  3 setTimeout(() => {  4   debugger;debug>

The debugger automatically breaks on the first executable line. To insteadrun until the first breakpoint (specified by adebugger statement), settheNODE_INSPECT_RESUME_ON_START environment variable to1.

$cat myscript.js// myscript.jsglobal.x = 5;setTimeout(() => {  debugger;  console.log('world');}, 1000);console.log('hello');$NODE_INSPECT_RESUME_ON_START=1 node inspect myscript.js< Debugger listening on ws://127.0.0.1:9229/f1ed133e-7876-495b-83ae-c32c6fc319c2< For help, see: https://nodejs.org/en/docs/inspector<connecting to 127.0.0.1:9229 ... ok< Debugger attached.<< hello<break in myscript.js:4  2 global.x = 5;  3 setTimeout(() => {>4   debugger;  5   console.log('world');  6 }, 1000);debug>nextbreak in myscript.js:5  3 setTimeout(() => {  4   debugger;>5   console.log('world');  6 }, 1000);  7 console.log('hello');debug>replPress Ctrl+C to leave debug repl>x5>2 + 24debug>next< world<break in myscript.js:6  4   debugger;  5   console.log('world');>6 }, 1000);  7 console.log('hello');  8debug>.exit$

Therepl command allows code to be evaluated remotely. Thenext commandsteps to the next line. Typehelp to see what other commands are available.

Pressingenter without typing a command will repeat the previous debuggercommand.

Watchers#

It is possible to watch expression and variable values while debugging. Onevery breakpoint, each expression from the watchers list will be evaluatedin the current context and displayed immediately before the breakpoint'ssource code listing.

To begin watching an expression, typewatch('my_expression'). The commandwatchers will print the active watchers. To remove a watcher, typeunwatch('my_expression').

Command reference#

Stepping#

  • cont,c: Continue execution
  • next,n: Step next
  • step,s: Step in
  • out,o: Step out
  • pause: Pause running code (like pause button in Developer Tools)

Breakpoints#

  • setBreakpoint(),sb(): Set breakpoint on current line
  • setBreakpoint(line),sb(line): Set breakpoint on specific line
  • setBreakpoint('fn()'),sb(...): Set breakpoint on a first statement infunction's body
  • setBreakpoint('script.js', 1),sb(...): Set breakpoint on first line ofscript.js
  • setBreakpoint('script.js', 1, 'num < 4'),sb(...): Set conditionalbreakpoint on first line ofscript.js that only breaks whennum < 4evaluates totrue
  • clearBreakpoint('script.js', 1),cb(...): Clear breakpoint inscript.json line 1

It is also possible to set a breakpoint in a file (module) thatis not loaded yet:

$node inspect main.js< Debugger listening on ws://127.0.0.1:9229/48a5b28a-550c-471b-b5e1-d13dd7165df9< For help, see: https://nodejs.org/en/docs/inspector<connecting to 127.0.0.1:9229 ... ok< Debugger attached.<Break on start in main.js:1>1 const mod = require('./mod.js');  2 mod.hello();  3 mod.hello();debug>setBreakpoint('mod.js', 22)Warning: script 'mod.js' was not loaded yet.debug>cbreak in mod.js:22 20 // USE OR OTHER DEALINGS IN THE SOFTWARE. 21>22 exports.hello =function() { 23   return 'hello from module'; 24 };debug>

It is also possible to set a conditional breakpoint that only breaks when agiven expression evaluates totrue:

$node inspect main.js< Debugger listening on ws://127.0.0.1:9229/ce24daa8-3816-44d4-b8ab-8273c8a66d35< For help, see: https://nodejs.org/en/docs/inspector<connecting to 127.0.0.1:9229 ... ok< Debugger attached.Break on start in main.js:7  5 }  6>7 addOne(10);  8 addOne(-1);  9debug>setBreakpoint('main.js', 4,'num < 0')  1 'use strict';  2  3 function addOne(num) {>4return num + 1;  5 }  6  7 addOne(10);  8 addOne(-1);  9debug>contbreak in main.js:4  2  3 function addOne(num) {>4return num + 1;  5 }  6debug>exec('num')-1debug>

Information#

  • backtrace,bt: Print backtrace of current execution frame
  • list(5): List scripts source code with 5 line context (5 lines before andafter)
  • watch(expr): Add expression to watch list
  • unwatch(expr): Remove expression from watch list
  • unwatch(index): Remove expression at specific index from watch list
  • watchers: List all watchers and their values (automatically listed on eachbreakpoint)
  • repl: Open debugger's repl for evaluation in debugging script's context
  • exec expr,p expr: Execute an expression in debugging script's context andprint its value
  • profile: Start CPU profiling session
  • profileEnd: Stop current CPU profiling session
  • profiles: List all completed CPU profiling sessions
  • profiles[n].save(filepath = 'node.cpuprofile'): Save CPU profiling sessionto disk as JSON
  • takeHeapSnapshot(filepath = 'node.heapsnapshot'): Take a heap snapshotand save to disk as JSON

Execution control#

  • run: Run script (automatically runs on debugger's start)
  • restart: Restart script
  • kill: Kill script

Various#

  • scripts: List all loaded scripts
  • version: Display V8's version

Advanced usage#

V8 inspector integration for Node.js#

V8 Inspector integration allows attaching Chrome DevTools to Node.jsinstances for debugging and profiling. It uses theChrome DevTools Protocol.

V8 Inspector can be enabled by passing the--inspect flag when starting aNode.js application. It is also possible to supply a custom port with that flag,e.g.--inspect=9222 will accept DevTools connections on port 9222.

Using the--inspect flag will execute the code immediately before debugger is connected.This means that the code will start running before you can start debugging, which mightnot be ideal if you want to debug from the very beginning.

In such cases, you have two alternatives:

  1. --inspect-wait flag: This flag will wait for debugger to be attached before executing the code.This allows you to start debugging right from the beginning of the execution.
  2. --inspect-brk flag: Unlike--inspect, this flag will break on the first line of the codeas soon as debugger is attached. This is useful when you want to debug the code step by stepfrom the very beginning, without any code execution prior to debugging.

So, when deciding between--inspect,--inspect-wait, and--inspect-brk, consider whether you wantthe code to start executing immediately, wait for debugger to be attached before execution,or break on the first line for step-by-step debugging.

$node --inspect index.jsDebugger listening on ws://127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29For help, see: https://nodejs.org/en/docs/inspector

(In the example above, the UUID dc9010dd-f8b8-4ac5-a510-c1a114ec7d29at the end of the URL is generated on the fly, it varies in differentdebugging sessions.)

If the Chrome browser is older than 66.0.3345.0,useinspector.html instead ofjs_app.html in the above URL.

Chrome DevTools doesn't support debuggingworker threads yet.ndb can be used to debug them.